A wind turbine transfers the energy of moving air into electrical energy. The moving air accelerates the rotor of the wind turbine. The rotation of the rotor is transferred to an electrical generator. The electrical generator transforms the rotational energy into electrical energy.
In the last years the concept of a direct driven wind turbine was established. In a direct driven wind turbine the rotational energy of the rotor is transferred to the generator directly without the use of a gearbox.
In a direct driven wind turbine the rotor of the wind turbine is directly connected to the rotor of the electrical generator. The chain of mechanically connected parts leading from the rotor of the wind turbine to the rotor of the generator is called the drive train of the wind turbine.
To allow the rotational movement and to provide the necessary stability of the rotating parts, the drive train is mounted with at least one bearing. This bearing allows the drive train to rotate. At the same time it provides the necessary stability by supporting the radial and axial loads and the bending moments present in the drive train.
WO 2011/003482 A2 describes a wind turbine main bearing realized to bear a shaft of a wind turbine. The bearing comprises a fluid bearing with a plurality of bearing pads. The document describes a bearing with a cylindrical bearing surface and a series of trust pads.
During the operation of the bearing foreign particles can enter the bearing or small pieces of the bearing pads might break off and stay in the bearing. These particles can cause defects or damages like scratches in the sliding surface of the bearing.
Defects in the sliding surface again lead to a higher friction that leads to a higher wear in the bearing. Therefore the sliding bearing has to be cleaned during service to remove loose particles from the bearing.
After some time of operation the bearing or parts of the bearing need to be exchanged during service because of damages in the sliding surface. A main bearing in a wind turbine especially in a direct driven wind turbine has a diameter of 2 meter or more and is heavy. This makes it very difficult to exchange it.
The exchange of a bearing in a wind turbine, especially a bearing in the drive train, is very complicated and costly. Often a crane has to be erected and the rotor of the wind turbine has to be dismantled. The Bearing is then exchanged and the rotor is mounted to the nacelle again. For other wind turbines, a crane is needed to lift a part of the drive train out of the nacelle of the wind turbine. The bearing or bearings are then exchanged and the drive train is mounted again.
Especially in the case of an off-shore installation this exchange is very expensive, as a specialized vessel is needed to perform the crane lift.
The bearings can then be refurbished in a plant specialized for this kind of work. The bearings can then also be reused.
Little defects like scratches have a negative effect on the lubrication and on the condition of the bearing pads. When the bearing shows too many of those defects, the bearing needs to be exchanged during service. The bearing can be refurbished and used again. The exchange of the bearing is very time consuming and expensive and needs to be avoided as long as possible.